1. Field of the Invention
The present invention relates to improvement of the structure of a semiconductor device having a bump electrode and particularly to a semiconductor device, having fine the bump electrode and having stable electrical characteristics, and its manufacturing method.
2. Description of the Related Art
In a semiconductor device, which is connected by TAB (Tape Automated Binding), a projection-shaped metallic electrode (hereinafter called bump electrode) is formed on an electrode pad. Normally, Al or an alloy containing Al is used as an electrode pad. However, when the bump electrode is directly formed on the electrode pad, there is a case in which A1 and the bump electrode react with each other to generate an intermetallic compound. In general, there is a high degree of risk in the use of the intermetallic compound, so that reliability of the device may be lost. Due to this, a plurality of thin metal film layers (hereinafter called as barrier metals) are normally formed between the electrode pad and the bump electrode to prevent Al and the bump electrode from reacting with each other to generate an alloy. The barrier metals are used to improve intensity of adhesion between the bump electrode and the electrode pad.
FIG. 1 is a cross sectional view showing the conventional bump electrode and its peripheral devices.
An electrode pad 7 is formed on a substrate 1 of a semiconductor integrated circuit. A passivation film 4, serving as an insulation film, is formed on the entire surface of the substrate 1 including the electrode pad 7. A passivation opening section which is formed at a predetermined position, is formed to expose the electrode pad 7. The opening section is formed of a side wall of an end portion of the passivation film 4 near the electrode pad 7 and an upper surface of the electrode pad, which is free of the passivation film 4. Moreover, the passivation opening section and the upper surface of the end portion of the passivation film 4 are covered with a barrier metal 3.
The barrier metal 3 is normally a two-layer structure having a first barrier metal 3a and a second barrier metal 3b. A bump electrode 6 is formed on the first barrier metal 3a.
The following will explain the conventional manufacturing method of the semiconductor device with reference to FIGS. 2 to 10.
FIG. 2 shows a process of forming the electrode pad 7.
A metal, serving as a metal electrode 7, such as an alloy of Al-Cu-Si is formed on the entire surface of the substrate 1. FIG. 3 shows a process of patterning the electrode pad 7.
The electrode pad 7 formed on the entire surface of the substrate 1 is etched by resist (not shown) to be patterned.
FIG. 4 shows a process of forming the passivation film 4.
The passivation film 4 is formed on the entire surface of the substrate 1 to completely cover the electrode pad 7.
FIG. 5 shows a process of forming the passivation opening section.
The passivation film 4 is etched such that a predetermined surface of the electrode pad 7 is exposed. A portion, which is surrounded with the exposed surface of the electrode pad 7 and the side surface of the passivation film 4 above the electrode pad, is the passivation opening section.
FIG. 6 shows a process of forming the barrier metal 3.
The barrier metal 3 is formed on the passivation film 4 and the passivation opening section by sputtering. The barrier metal 3 is normally a two-layer structure comprising a first barrier metal 3a and a second barrier metal 3b.
In a case where Au is used as a bump electrode (not shown) formed on the first barrier metal 3a in a later process, as an example of the barrier metal 3, there are used a sputtering thin film of Ni having a thickness of 300 nm, serving as a first barrier metal 3a, and a sputtering thin film of Ti having a thickness of 100 nm, serving as a second barrier metal 3b.
FIG. 7 shows a process of patterning resist 5.
First, the entire upper surface of the first barrier metal 3a is coated with resist 5. Then, resist 5 on the electrode pad 7 is removed to be opened such that the surface of the first barrier metal 3a of the upper portion of the electrode pad 7 is exposed. A portion, which is surrounded with the exposed surface of the first barrier metal 3a and the wall of the opening of resist 5, is used as a resist opening section.
FIG. 8 shows a process of forming the bump electrode 6.
The bump electrode 6 is formed on the resist opening section by electrolytic plating with the first barrier metal 3a as a plating electrode. The height of the bump electrode 6 is generally 10 to 20 .mu.m.
FIG. 9 shows a process of removing resist 5.
Resist 5 is removed such that the bump electrode 6 is left.
FIG. 10 shows a process of patterning the barrier metal 3.
The barrier metal 3 around the bump electrode 6 is partially removed by wet etching, and the barrier metal 3 is left under the bump electrode 6.
As material of the barrier metal 3, Ni and Ti are used. Ni is etched with a mixed solution of HNO.sub.3, HCl, and CH.sub.3 COOH and Ti is etched with a water solution of HF.
The above is the outline of the conventional semiconductor device and its manufacturing method. However, in the conventional manufacturing method, there is a problem in that a large side etch portion is generated in the process of removing the barrier metal 3 of FIG. 10.
The side etch portion is a portion, which is surrounded with the bump electrode 6 adjacent barrier metal 3, the end portion of the barrier metal 3, and the portion of the passivation film 4 adjacent barrier metal 3.
The generation of the side etch portion is a phenomenon in which etching is advanced from the peripheral portion of the bump electrode 6 to the portion just under the bump electrode 6 in addition to the barrier metal 3 exposed onto the surface and a over-hang shape is generated. This phenomenon is caused when the etching for removing the barrier metal 3 is an isotropic etching using corrosive liquid, and the etching rate of the bump electrode 6 and that of the barrier metal 3 are different.
If the side etch portion is advanced up to the passivation opening section, there occurs a phenomenon in which the electrode pad 7 corrodes and the electrical connection between the electrode pad 7 and the bump electrode 6 is broken. This phenomenon produced a detective contact, and reliability of the semiconductor device is considerably reduced.
Due to this, in the manufacturing process, the size of the bump electrode 6 is larger than the passivation opening section to have an allowable positioning variation. However, the size of the bump electrode must be set to be more than necessary in order to prevent generation of a defective contact due to the side etch.
The point of making the size of the bump electrode 6 larger than the passivation opening section has recently become an obstacle to the need of reducing the size of the bump electrode 6.
In order to eliminate the above-mentioned side etch portion, there is a method in which the barrier metal 3 is coated with resist 5 again just before being removed after the bump electrode 6 is formed, and the etching of the barrier metal 3 is performed by the resist patterning.
Moreover, there is a method of removing the side etch by dry-etching using plasma in place of the wet system using the solution.
However, in the former method, there are difficulties in the resist coating after the bump electrode 5 is formed and the removal of resist 5, and such coating and removal become complicated. In the latter method, though the number of processes is the same as the conventional case, a dry-etching device is expensive, and the the number of through puts is reduced since vacuum is introduced in the process, so that the manufacturing cost is increased.
As mentioned above, in the conventional semiconductor device, the size of the bump electrode must be enlarged in order to avoid the defective contact, and so that unfavorable influence is exerted on reducing the size of the bump electrode.